(1) Field of the Invention
The present invention relates to a method for producing a battery. More particularly, the present invention relates to a method for producing a battery of high reliability which uses a laminated film as the casing.
(2) Description of the Prior Art
In recent years, portable type electronic appliances have become smaller in size and lighter in weight. In this connection, the batteries used therein as an energy source are required to follow the same trend. The lithium secondary battery, which is currently popular among the above batteries, uses an electrolytic solution and therefore needs a rigid casing (e.g. a metal can) for leakage protection, etc.; therefore, the battery is thought to have a limit in weight reduction or thickness reduction. In order to realize the weight reduction and/or thickness reduction of the battery, it is necessary to use a polymer gel or the like in place of an electrolytic solution and further use a casing other than a metal can. As a casing used for such a purpose, there is ordinarily used a laminated film obtained by laminating (1) an aluminum foil having a higher resistance to the transmission of water or organic electrolytic solution than polymer resin films have and (2) a polymer resin film. In such a laminated film, a synthetic resin film with high heat-sealability, such as polyethylene film, polypropylene film or the like is used as a sealing layer; and a film such as polyester film or the like is used as a protective layer. In sealing the battery, a battery power source comprising a positive electrode, a negative electrode and an electrolyte is interposed between two laminated films and the peripheral areas of the two laminated films surrounding the battery power source are heat sealed with each other using a heating means.
The conventional sealing of a battery, which comprises pressure-bonding a polyethylene, a polypropylene or the like only by heat sealing has a problem in that the sealed area formed is insufficient in heat resistance at temperatures of 100xc2x0 C. or higher and gives rise to peeling caused by resin softening, etc. There is also a problem in that the sealing strength of the sealed area is low even at ordinary temperature and the sealed area gives rise to peeling when the internal pressure of battery rises owing to, for example, the expansion of battery power source or the generation of gas inside battery; in an extreme case, leakage of electrolytic solution takes place and the reliability and safety of battery decrease.
It is disclosed in JP-A-11-86807 that when a positive electrode, a negative electrode, an electrolytic solution, etc. are placed in a bag made of a laminated sheet of a metal foil and a plastic layer, the bag is heat-sealed, and the sealed bag is subjected to a radiation treatment, the resulting bag shows effective prevention of liquid leakage. The literature describes, as the method for radiation treatment, application of radiation to a cardboard box containing batteries or to batteries arranged on a conveyor. The literature also describes that as the radiation, not only a xcex3-ray but also an electron beam can be used; however, no specific example is given about the use of an electron beam in the literature.
An investigation by the present inventor indicates that, in the technique described in JP-A-11-86807, the irradiation of a battery power source with an electron beam causes decomposition of electrolytic solution, etc., resulting in reduction in battery performance. Further, no sufficient adhesion is obtained at the parts where the lead electrode extending from a positive electrode or a negative electrode and a laminated film are adhered to each other (these parts are sites at which liquid leakage occurs most easily).
Also, JP-A-7-78604 discloses a battery structure obtained by interposing a battery power source between two sheet-shaped terminals also functioning as a positive electrode current collector and a negative electrode current collector, via a sealing material and subjecting the sealing material to heat sealing; the heat sealed layer has improved heat resistance when, as the sealing material, a modified polyethylene beforehand irradiated with an electron beam is used. This technique relates to the modification of a sealing material alone and is different from a technique for improvement of adhesivity and sealing strength in a battery structure obtained by sealing a battery power source using a laminated film as the casing.
It is known to form, in a battery, a safety valve for releasing a gas outside, in order to prevent the bursting of battery when the gas pressure inside battery has increased. It is described in, for example, JP-A-11-86823 to form, in the sealed area of a battery structure using a laminated film as the casing, a part having a pressure resistance lower than that of the other major sealed part so that the gas release from the part of lower pressure resistance is made possible when the pressure inside battery has increased. In this technique of forming a part of lower pressure resistance in a sealed area, a material different from that of other major sealed part need be used for the part of lower pressure resistance, making the battery production complicated; further, since the whole sealed area is formed by heat sealing, the overall heat resistance is insufficient.
In view of the above-mentioned problems of the prior art, the objective of the present invention is to provide a method for producing a battery using a laminated film as the casing, which battery has a high sealing strength, particularly a high peeling strength at the terminals of leading electrodes without sacrificing battery performances and therefore has high reliability and safety.
Another objective of the present invention is to provide a method for producing, at high productivity and easily, a battery using a laminated film as the casing, which battery has high strength at the whole sealed area, is provided with a safety mechanism allowing gas release from a particular site when the pressure inside battery has increased owing to the happening of an abnormality, and has high reliability and safety.
Accordingly, the present invention is directed to a method for producing a battery, comprising a battery power source having a positive electrode, a negative electrode and an electrolyte, and a laminated film of a metal foil and a polymer resin layer as a casing encapsulating the battery power source therein;
the method comprising the steps of:
heat sealing the sealing area of the laminated film, whereby encapsulating the battery power source in the laminated film; and
irradiating, with an electron beam, a portion of the heat sealed area, thereby the irradiated area crosses, in the direction along the peripheries of the heat sealed area, the overlapping areas of the heat sealed area and the leading electrodes projecting toward outside from the positive electrode and the negative electrode.
By thus irradiating the heat sealed area of the laminated film with an electron beam, a crosslinked structure is formed in the heat sealed area. Thereby, the resin in the laminated film has improved heat resistance and also has improved adhesivity, i.e. improved sealing strength. That is, in the area where the two films have been heat sealed and welded to each other, the resins of the two films are thermally melted, mixed and, moreover, bonded strongly by crosslinking. By irradiating, in particular, the overlapping areas of the heat sealed area and the leading electrodes, with an electron beam, the parts of the polymer resin layer contacting with the leading electrodes can have improved heat resistance and improved sealing strength; as a result, it is possible to prevent liquid leakage and produce a battery having high reliability.
Another aspect of the present invention is directed to a method for producing a battery, comprising a battery power source having a positive electrode, a negative electrode and an electrolyte, and a laminated film of a metal foil and a polymer resin layer as a casing encapsulating the battery power source therein;
the method comprising the steps of:
heat sealing the sealing area of the laminated film; and
irradiating at least part of the heat sealed area of the laminated film with an electron beam while preventing the battery power source from being irradiated with the electron beam.
By thus preventing the battery power source from being irradiated with an electron beam, there can be produced a battery which has a high sealing strength, particularly a high peeling strength at the terminals of leading electrodes without sacrificing the battery performances, even when there is used an electrolyte containing, for example, a salt easily decomposing upon irradiation with an electron beam, such as LiPF6.
When the irradiation with an electron beam is conducted to the whole length in the direction along peripheries of-the heat sealed area surrounding the battery power source, the whole circumference of battery has improved sealing strength and a battery of high reliability can be produced.
In the present invention, a portion of the heat sealed area in the direction along the peripheries of the heat sealed area surrounding battery power source may not be irradiated with an electron beam. By thus allowing a portion of the sealed area not to be irradiated with an electron beam, this part (electron beam-unirradiated part) has lower peeling strength than the electron beam-irradiated part and, once abnormal pressure increase arises inside the battery, causes peeling preferentially; therefore, the part can be used as a safety mechanism for gas release. This is advantageous in that the peeling position, that is, the gas releasing position becomes predictable. The present method can produce a battery having such a safety mechanism, at high productivity.